FIELD OF THE INVENTION
The present invention relates to a process of condensing phthalic anhydride with an alkylbenzene to form an alkylbenzoylbenzoic acid, followed by ring closure to form an alkylanthraquinone.
Alkylanthraquinones find utility in the well-known working solution process for preparing hydrogen peroxide in which a supported metal, such as palladium, catalyst is used to reduce the alkylanthraquinone to the corresponding alkylanthrahydroquinone. These supported metal catalysts are readily poisoned by impurities including those often associated with the production of alkylanthraquinones. Normally these impurities are removed during the manufacture of alkylanthraquinones by the time consuming and costly procedures of vacuum distillation or crystallization.
Alkylbenzoylbenzoic acids, which are the starting material for the process of the present invention, are customarily prepared by a Friedel-Crafts reaction from phthalic anhydride and an alkylbenzene. The reaction is usually performed in an inert solvent of the type commonly referred to as Friedel-Crafts solvents such as chlorobenzene, dichlorobenzene, trichlorobenzene, tetrachloroethane, chloroform, carbon tetrachloride, carbon disulfide, etc.
Generally the alkylbenzoylbenzoic acid is converted to the corresponding alkylanthraquinone by heating it in sulfuric acid or oleum for from three to five hours. Stoichiometry is critical as many side reactions are possible. Furthermore, both the starting material and product can be sulfonated. The intermediate carbonium ion which is formed can ring close to the desired product or alkylate neutral species leading to high boiling tarry substances. Conditions must be balanced to optimize yield and minimize side reactions.
U.S. Pat. No. 3,032,560 discloses the preparation of amylanthraquinone by heating one part of 2(4'-amylbenzoyl)benzoic acid at from 65.degree. to 95.degree. C. in the presence of at least 5 parts of oleum containing from 5 to 10% sulfur trioxide.
This type of process is characterized by low yields, 71%, probably because of poor dispersion of the solid in the oleum producing zones of unfavorable stoichiometry which leads to tarry by-product formation. Purification of the alkylanthraquinone crude by distillation or crystallization is necessary to remove these tarry by-products.
U.S. Pat. No. 4,035,396 discloses preparing alkylanthraquinones by heating at 85.degree. to 95.degree. C. one part of an alkylbenzoybenzoic acid dissolved in at least five parts of trichlorobenzene in the presence of oleum. This process is an improvement over the above process since the solvent provides a more uniform dispersion of the alkylbenzoylbenzoic in the sulfuric acid or oleum and a more uniform stoichiometry is achieved. This process usually requires a high boiling solvent which exerts a high solvating action on the alkylbenzoylbenzoic acid. A chlorinated aromatic such as trichlorobenzene is commonly used as the solvent. However, this and similar solvents can be sulfonated and react in other ways with the oleum and reactants to produce tars. Purification by distillation or crystallization is necessary to remove these tars.